Fertile research field
Publication Date: Wednesday Oct 30, 1996

Fertile research field

Scientist studies cooperative alliance between plants and bacteria

@By:by Heather Rock Woods Sharon Long grows alfalfa and peas.

Her family tree includes farmers, but Long is a professor of biological sciences at Stanford, and she studies how those plants cooperate with helpful bacteria.

She sees her research as detective work in a way, talking about mysteries and clues and staying focused on the most important questions to pursue, whether answering them requires using genetics, chemistry or molecular techniques.

The mystery that Long has been unraveling for some 15 years is how the legume family of plants and a bacteria with special properties sustain their intimate, healthy relationship.

All living things need nitrogen to make amino acids--the building blocks of proteins. But only a small percentage of bacteria--and no plants--can take advantage of the plentiful nitrogen in the atmosphere because it is in a stable form that rarely mixes with other molecules. Rhizobium is one of the bacteria that can pull inert nitrogen from the air and turn it into ammonia, a chemical that plants can absorb through their roots.

"The key atom, which is hard to (get), is nitrogen," said Long, who is also a Howard Hughes Medical Institute investigator.

While some plants use nitrate, another form of nitrogen found in some soils, the legume family has harnessed Rhizobium to make usable nitrogen. In return, the alfalfa, wisteria, acacia trees, mesquite, beans, lupine and other legume species let the bacteria eat sugar made by photosynthesis.

Legumes "can make a big difference to other plants, to turn nitrogen back into the soil," Long said.

What she has discovered over the years is that alfalfa sends a chemical signal to nearby bacteria in the soil. Rhizobium sends a return signal, that tells the plant to grow tumor-like nodules on its roots.

"The bacteria provoke all this to happen while they're still on the outside," Long said.

Then the bacteria moves into the nodules, and fixes nitrogen into ammonia while feeding on the sugars alfalfa makes.

"The benefit to the plant is getting the nitrogen nutrition it needs without having nitrogen fertilizer," Long said.

This symbiotic relationship could prove useful for making fertilizers with less energy. Currently, nitrogen fertilizer is made by burning nitrogen gas and hydrogen gas at extremely high temperatures and pressures. But in 50 years, natural gas may be too expensive and scarce for Third World countries to afford fertilizer, Long said.

"That's why biological nitrogen fixing is something the Department of Energy is very interested in. It's important to know more about it. But there's no guarantee that basic research will help."

One avenue commercial and academic researchers are exploring is making the legumes and Rhizobium more efficient, giving farmers a bigger source of nitrogen when they plow legumes into the field, a practice that goes back 2,000 to 3,000 years, Long said.

Long's research has won her many awards, most notably a MacArthur "genius" fellowship in 1992. She is using the part of the award money--$50,000 a year for five years--to collaborate with a British colleague on investigating a species of Rhizobium that makes a protein made by no other Rhizobium species. The protein "may assist the communication of the plant and bacteria," Long said.

"What's exciting and mysterious in the field right now is . . . the way the bacteria cause the plant to form nodules. What is it in the plant that causes it to smell this signal (from Rhizobium)?" Long said.

Right now she is intentionally growing a mutant crop of alfalfa, with individual plants missing different genes. A mutant plant that doesn't respond to the bacterial signal will give Long clues to what necessary factor is missing.

Long's lab and a group in France independently nailed down the bacterial signal in 1990. The signal is a molecule with a fatty tail at one end at a sulfate group at the other, "which makes it work on alfalfa."

About four years earlier, when she discovered that the legumes also send a signal, Long's lab identified the signal, a kind of flavonoid.

Flavonoids are compounds made by plants that, among other things, give color to eggplant skin and red roses. "When we proved the signal was a flavonoid, it was the first clue that anyone had of what it was for (besides pigment)," Long said.

While growing is a common thread in Long's life--her husband Harold McGee gardens and has written books on the science of food and cooking-- Long said she didn't get into her field because of her farming predecessors.

Long shifted into the research after earning a graduate degree in developmental biology from Yale, because she read some articles about Rhizobium influencing the development of nodules in legumes, and it "appeared to be an interesting case of development due to the interaction with another organism."

"I feel very lucky that I happened to get interested in plants, because they're so interesting."

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